1. Field of the Invention
The present invention relates to the hydraulic control system of an automatic transmission for a vehicle and, more particularly, to hydraulic control system for a four-speed automatic transmission which can enhance shift quality by alleviating shock caused by gear shifting and can improve responsiveness with respect to the gear shifting.
2. Description of Related Art
In general, an automatic transmission provides a necessary gear ratio to operate the vehicle under wide range of speeds and loads. It does this with a minimum amount of effort on the part of the driver. That is, automatic upshifts and downshifts are a convenience for the driver because a foot-operated clutch is not required to shift gears and because the vehicle can be brought to a stop without the use of a clutch and without shifting the transmission into neutral.
A conventional automatic transmission for a vehicle comprises a torque converter having an impeller, a turbine and a stator, a gear train connected to the torque converter to provide various forward speed ranges and reverse, a plurality of friction members such as disc clutches, one-way clutches which control gear action, and a hydraulic control system for controlling the operation of the friction members.
One example of a conventional hydraulic pressure control system of an automatic transmission is illustrated in FIG. 7. The conventional hydraulic control system comprises a damper clutch control valve 14 for controlling the operation of a damper clutch DC mounted inside torque converter 10 to increase power train efficiency, pressure regulator valve 20 for regulating a hydraulic pressure generated from hydraulic pump 18 according to the automatic transmission requirements, and reducing valve 19 for feeding a stable pressure to solenoid valve 12 and the damper clutch control valve 14.
Manual valve 26, which is connected to an outlet of hydraulic pump 18 to receive line pressure therefrom, is designed to deliver a line pressure to the pressure regulator valve 20 and shift control valve 24. The manual valve 26 moves in accordance with a position(range) of a shift selector lever having five positions(ranges) P, R, N, D, 2 and L.
Shift control valve 24, which is actuated in response to two shift control solenoid valves SA and SB controlled by transmission control unit TCU, is designed to feed hydraulic pressure selectively through a first-to-second speed shift valve 28, end clutch valve 30, second-to-third and third-to-fourth speed shift valve 32, and rear clutch exhaust valve 34 to a front clutch C1, a rear clutch C2, low-reverse brake B2, kick down servo band brake B1, and end clutch C3.
N-D control valve 36 for alleviating shock caused by shifting the shift selector lever from the neutral "N" range to the drive "D" range is connected to the rear clutch C2 via the rear clutch exhaust valve 34. N-R control valve 38 for reducing shock caused by shifting the shift selector lever from the neutral "N" range to the reverse "R" range is connected to low-reverse brake B2 via first-to-second speed shift valve 28.
Also, pressure control solenoid valve 22 is connected to pressure control valve 21 to reduce shock caused by gear shifting.
Shifting operation of this conventional hydraulic control system will be briefly described hereinafter.
When the drive "D" range is selected by the shift selector lever, hydraulic pressure generated from the hydraulic pump 18 is fed to the manual valve 26 along a line L1. The hydraulic pressure supplied to the manual valve 26 flows along lines L2 and D1.
In a first speed ratio at the drive "D" range, both of the shift control solenoid valves SA and SB are controlled to be "ON" by transmission control unit TCU, and therefore the hydraulic pressure flowing along line L2 is exhausted through each exit ports of shift control solenoid valves SA and SB to have no effect on the position of the valve spool of shift control valve 24. At the same time, pressure control solenoid valve 22 is controlled by transmission control unit TCU to be "ON" such that the hydraulic pressure flowing along line L3 via reducing valve 19 is exhausted through an exit ports of the pressure control solenoid valves 22.
Since the hydraulic pressure in the line D1 is not fed to the first-to-second speed shift valve 28, the hydraulic pressure flowing along the line D1 is supplied to rear clutch C2 via rear clutch exhaust valve 34 to apply the same.
In a second speed ratio at the drive "D" range, only the shift control solenoid valve SA is controlled by transmission control unit TCU to be "OFF" such that the hydraulic pressure is exhausted through shift control solenoid valve SB to thereby move the valve spool and plug of the shift control valve 24 rightward. As a result, the hydraulic pressure from the manual valve 26 flows along line D2.
Accordingly, the hydraulic pressure in line D2 is fed to the left side of first-to-second speed shift valve 28 to push the valve spool thereof rightward. At this point, since pressure control solenoid valve 22 is controlled to be "OFF" to prevent the hydraulic pressure from being exhausted, and thus the hydraulic pressure from the hydraulic pump 18 is supplied to the left side of pressure control valve 21 via reducing valve 19 to push the valve spool therein rightward. Accordingly, the hydraulic pressure passing through the line D1 is supplied to the first-to-second speed shift valve 28 via the pressure control valve 21.
At this point, since the valve spool of first-to-second speed shift valve 28 has been pushed rightward, the hydraulic pressure which has passed through pressure control valve 21 is supplied to an apply chamber of kick-down servo band brake B1 to apply the same, thereby accomplishing the second speed ratio.
In third speed ratio at the drive "D" range, because the transmission control unit TCU makes both of the shift control solenoid valves A and B switch "OFF", the hydraulic pressure is prevented from being exhausted. Thus, the valve spool of the shift control valve 24 is moved rightward farther and the valve plug is stopped by means of a stopper.
At this state, because lines D2 and D3 are opened simultaneously, a portion of the hydraulic pressure flowing along line D3 is fed to the right side of end clutch, valve 30 to push the valve plug leftward and is then fed to the end clutch C3 to apply the same. The other portion of the hydraulic: pressure flowing along the line D3 is fed to the left side of second-to-third and fourth-to-third speed shift valve 32 to force the valve spool rightward.
Further, the hydraulic pressure passing through first-to-second speed shift valve 28 via pressure control valve 21 is fed to second-to-third and fourth-to-third speed shift valve 32. At this point, since the valve spool of second-to-third and fourth-to-third speed shift valve has been pushed rightward, a portion of the hydraulic pressure supplied to the second-to-third and fourth-to-third speed shift valve 32 is fed to a release chamber of kick down servo band brake B1 and another portion of the hydraulic pressure is fed to the front clutch C1.
Accordingly, the kick down servo band brake B1 which has been applied in tile second speed ratio is released and front clutch C1 is applied, thereby accomplishing the third speed ratio.
In a fourth speed ratio at the drive "D" range, because transmission control unit TCU makes only the shift control solenoid valve SB switch "OFF", the valve spool of shift control valve 24 moves rightward farther than it has been at the third speed ratio to open the fourth line D4. When this occurs, the hydraulic pressure supplied to the left side of the rear clutch exhaust valve 34 pushes the valve spool rightward to interrupt the hydraulic pressure for applying the front clutch C1 and the hydraulic pressure for releasing kick down servo band brake B1. Accordingly, kick down servo band brake B1 is applied again automatically and end clutch C3 remains in the applied state, thereby accomplishing the fourth speed ratio.
When the shift selector lever is set at the reverse "R" range, the hydraulic pressure from the manual valve 26 is supplied to the right side of second-to-third and fourth-to-third speed shift valve 11 via rear clutch exhaust valve 34 to force the valve spool leftward, and thus the hydraulic pressure from manual valve 26 is supplied to the front clutch C1 and the low reverse brake B2 and acts to release the kick-down servo band brake B1 to reverse the vehicle.
As described above, front clutch, rear clutch and end clutch C1, C2 and C3 respectively, are applied in the third speed ratio in the drive "D" range, and rear clutch and kick down servo band brake C2 and B1 are applied in the second speed ratio at the drive "D" range. Therefore, to accomplish a downshift from the third speed ratio to the second speed ratio, front clutch and end clutch C1 and C3 should be released. However, since the pressure applied front clutch C1 and to the release chamber of kick down servo band brake B1 flows along same line, the release pressure of front clutch C1 should be supplied at the time the apply pressure of the front clutch C1 is exhausted. If the apply pressure of front clutch C1 is not quickly exhausted, the apply pressure of kick down servo band brake B1 is fed to the kick down servo band brake such that the shift shock occurs. Complete of exhaustion of the apply pressure of front clutch C1 and supplying the apply pressure for the kick down servo band brake should be accomplished in 0.6 second. However, it is very difficult to control the friction members in turn in a such short time.
Additionally, since the conventional hydraulic control system as described above can not provide a skip shift, e.g. from the second speed ratio to the fourth speed ratio or from the fourth speed ratio to the second speed ratio, responsiveness with respect to a gear shift is retarded.